1 Research Update No. 16, December 2020, revised July 2021 India’s break-out from Gondwana – how to escape from a confined space Context This note attempts to convey the thinking behind the story of India’s initial break-out from Gondwana in the Early Cretaceous as portrayed in our geometrical models. Whereas the problem may, at first sight, appear to be lacking in constraints (such as marine magnetic anomalies), careful thinking and iterative modelling shows that the number of distinct options is actually very limited. There is, we argue, little room for manoeuvre within existing data without inventing unlikely scenarios or violating basic rules of global tectonics, such as (1) no overlapping of continental landmasses and (2) ensuring steady forward growth of ocean crust, i.e. without consumption of new ocean crust once created. India is initially surrounded by continental masses on three sides, restricting many options. The story is supported by paleomaps presented at the end of the text and an animation that can be found on the website www.reeves.nl. The initial draft of this Update was completed late in 2020 and predated by only a few weeks the adoption of the 2020 Geological Time Scale in all our work. Its release was delayed while the model was reworked in the first six months of 2021. The model of late-2020 (CR20BBJL) differs only in details from the current one (CR21BBHQ). The refinements of 2021 reinforce the earlier concepts. Figure 1 shows the paths of a point common to Africa, Antarctica and India respectively that were originally identical in our Gondwana reassembly, off the southern tip of India. The paths are plotted against our fixed- mantle-plume reference frame. Despite the change in time scale and subsequent adjustments to the model, both old and new models show several robust features. The northward path of Africa is fairly consistent. Four events emerge for the paths of Antarctica and India (Figure 1): A. At about184 Ma East Gondwana (Antarctica and India) start to separate from West Gondwana (Africa). B. At about 155 Ma, Antarctica and India start moving southward more quickly than they were previously following Africa loosely to the north. C. At 143 Ma, India starts to separate from Antarctica but still follows it more closely than it does Africa. D. Around 117 Ma, when Madagascar joins Africa, India stops following Antarctica southwards and starts moving to the NE. This movement accelerates markedly after about 90 Ma. The thinking behind model development has followed similar paths throughout. Several iterations of the plate circuit have refined some details, the most recent (CR21BBHQ) trying more carefully to delay unsubstantiated movement between India and Madagascar as long as possible without offending other constraints. Since this India-Madagascar connection is the closure of the plate circuit, it has taken much
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1
Research Update No. 16, December 2020, revised July 2021
India’s break-out from Gondwana –
how to escape from a confined space
Context
This note attempts to convey the thinking behind the story of India’s initial break-out from Gondwana in the
Early Cretaceous as portrayed in our geometrical models. Whereas the problem may, at first sight, appear to
be lacking in constraints (such as marine magnetic anomalies), careful thinking and iterative modelling shows
that the number of distinct options is actually very limited. There is, we argue, little room for manoeuvre
within existing data without inventing unlikely scenarios or violating basic rules of global tectonics, such as
(1) no overlapping of continental landmasses and (2) ensuring steady forward growth of ocean crust, i.e.
without consumption of new ocean crust once created. India is initially surrounded by continental masses
on three sides, restricting many options. The story is supported by paleomaps presented at the end of the
text and an animation that can be found on the website www.reeves.nl.
The initial draft of this Update was completed late in 2020 and predated by only a few weeks the adoption of
the 2020 Geological Time Scale in all our work. Its release was delayed while the model was reworked in the
first six months of 2021. The model of late-2020 (CR20BBJL) differs only in details from the current one
(CR21BBHQ). The refinements of 2021 reinforce the earlier concepts.
Figure 1 shows the paths of a point common to Africa, Antarctica and India respectively that were originally
identical in our Gondwana reassembly, off the southern tip of India. The paths are plotted against our fixed-
mantle-plume reference frame. Despite the change in time scale and subsequent adjustments to the model,
both old and new models show several robust features. The northward path of Africa is fairly consistent.
Four events emerge for the paths of Antarctica and India (Figure 1):
A. At about184 Ma East Gondwana (Antarctica and India) start to separate from West Gondwana
(Africa).
B. At about 155 Ma, Antarctica and India start moving southward more quickly than they were
previously following Africa loosely to the north.
C. At 143 Ma, India starts to separate from Antarctica but still follows it more closely than it does
Africa.
D. Around 117 Ma, when Madagascar joins Africa, India stops following Antarctica southwards and
starts moving to the NE. This movement accelerates markedly after about 90 Ma.
The thinking behind model development has followed similar paths throughout. Several iterations of the
plate circuit have refined some details, the most recent (CR21BBHQ) trying more carefully to delay
unsubstantiated movement between India and Madagascar as long as possible without offending other
constraints. Since this India-Madagascar connection is the closure of the plate circuit, it has taken much
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effort to ensure that errors and uncertainties are placed where data is least reliable at all times. The
following explanation from December 2020 has been only lightly edited in June 2021.
Jurassic times
We start with the Gondwana reassembly published in Research Update No 14 (September 2020) and
consider the separation of East Gondwana (EGO) from West Gondwana (WGO) in the interval 184.2 to 154.9
Ma (GTS2020). We retain India, along with Madagascar, in their reassembly position as part of EGO in this
interval. Evidence to support this includes the similar distances between pairs of early marine magnetic
anomalies on the Africa plate off Somalia and Mozambique (WGO) and their conjugates off Madagascar and
Antarctica respectively. Our tight reassembly of East Gondwana must be retained for at least 29 myr for
these distances to remain closely similar. We are also not aware of any evidence of rifting within East
Gondwana during Early and Middle Jurassic times. (When was the earliest rifting between Australia and
Antarctica? In this model, Australia remains fixed to Antarctica until 145 Ma.)
The spreading direction between EGO and WGO changes markedly at about 154.94 Ma (GTS2020) with EGO
turning to a more southerly-oriented path against WGO. The position of Antarctica with respect to Africa is
well constrained by marine magnetic anomalies (Mueller & Jokat, 2019) after this time but the anomalies
between Madagascar and east Africa are less well defined. While the hidden geology off the east coast of
Figure 1. Paths of a point common to Africa (blue), India (green) and Antarctica (grey) plotted on the global mantle
plume reference frame. The centre of curvature (i.e. Euler interval pole) of the Davie FZ and the proto-Owen FZ both
lie on the fault off Western Australia, as shown by the concentric circles. This point, anchored to the India plate,
progresses northward along this Australian fault line (shown by red circles) while the Davie and Owen FZs are active in
pure strike-slip mode, 143-117 Ma. The main events, A, B, C and D are described in the text. Note that, once
Madagascar comes to rest as part of the Africa plate (the timing of the figure), the fault off NE Madagascar has the
same geometry as the by-then-defunct Davie FZ.
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Madagascar and the west coast of India are both little explored, we have admitted the possibility of a small
rifting event between the two starting at this time with the ‘risk’ of Madagascar becoming isolated from
both parts of Gondwana. By 142.3 Ma (M18, GTS2020) we have allowed 68 km of extension at the north
end of the rift (off the Bombay High), tapering to zero at the south end of Madagascar, the rifting being
slightly transtensional.
At around 143 Ma (Jurassic-Cretaceous boundary) the long, straight, N-S oriented transform separating the
Madagascar plate from the Africa plate since 155 Ma had to start morphing into the familiar, arcuate Davie
FZ. The arcuate DFZ then defines the long path of Madagascar’s remaining trajectory against Africa until it
reaches it stationary position at about 117 Ma. Our model has India and Australia still in their ‘fit’ position
against Antarctica until 142.3 Ma. This may need some revision when more recent interpretation of the
Australia-Antarctica rifting history is obtained.
At 142.3 Ma (M18, begin Berriasian, GTS2020) our model starts, slowly at first, opening the rift between
India (including greater India) and Antarctica-Australia. At this time, the position of Antarctica against Africa
is well constrained by marine magnetic anomaly data. A lower degree of certainty confines the position of
Madagascar. Nevertheless, it seems certain that the position of India between Madagascar and Antarctica
can be little different from the original Gondwana ‘fit’ positions of the three. Separation of India from
Australia-Antarctica must be limited on account of little room for movement relative to all its neighbouring
fragments at what is now the southern end of India; published interpretations of the positions of
Madagascar against Africa and of Antarctica against Africa at this time allow only just enough room for
southernmost India and Sri Lanka between Madagascar and Antarctica. Advocates of looser fits should note,
therefore, that even our tight ‘fit’ for East Gondwana may not actually be quite tight enough.
At the other end of (Greater) India, 3400 km to the east, the movement of Greater India against Australia
must lead to the creation of the prominent transform fault running NW from the west coast of Australia.
Marine magnetic anomalies here, off Perth, punctuate - and so constrain - the timing of this opening. A
great circle drawn perpendicular to the fault off WA must pass through the initial instantaneous rotation
pole that defines strike-slip on this transform (Figure 2). The line passes through southernmost Madagascar.
The SE coast of India, south of the Krishna-Godavari delta, and the eastern coastline of Sri Lanka form a
straight fault line against the conjugate margin of Antarctica in our reassembly. A great circle normal to this
second line also passes through southern Madagascar (Figure 2).
Figure 2. Pure strike-
slip on the two (red)
fault features defines
an initial
instantaneous
rotation pole for
India against
Antarctica that leads
to extension along
the whole of the
India-Antarctica
margin with
extension reducing
from east to west.
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Our model has slow anti-clockwise rotation for India with respect to Antarctica about a pole near here (i.e.
rifting between the east coast of India and Antarctica) from the start of the Cretaceous. Note from Figure 2
that, if India and Madagascar retain their relative positions, Madagascar will put the Davie FZ into
compression with this initial rotation. Eventually (130-125 Ma, approx.), dextral strike-slip on the Mwembishi
FZ crossing Africa (Reeves and Teasdale, 2019; https://www.reeves.nl/upload/The-creation-of-the-African-
margins-revised.pdf ) will make more room for Madagascar to rotate in this fashion.
Early Cretaceous
By 130.68 Ma (M7, Hauterivian, GTS2016) we have opened India (with Greater India/Tibet firmly attached)
away from Antarctica-Australia such that the M7 anomaly off Western Australia is equidistant from the
margins of Australia and Greater India with pure strike-slip on the fault off WA. The coastal fault off SE India
is still slightly transtensional and Madagascar has advanced southwards on the arc of the Davie FZ to exactly
keep pace with India, i.e. no (further) rifting between it and India in this period. Note that the rotational
movement of India gives dextral strike slip on the fault off WA as well as on the transform separating the
ocean off NW India from that off Somalia. The latter contact line will, much later, develop into the sinistral
Owen FZ but is dextral initially. India is thus able to rotate within the confines of the other continents that
surround it to the east and west while opening a new ocean between itself and Antarctica. The
development of Neo-Tethys continues to India’s north.
This mechanism persists with only a slight change of rotation pole until 121.4 Ma (M0, Barremian, GTS2020).
By this time, Madagascar has almost reached the end of its southward travel against Africa. The movement
of Madagascar against Africa is still confined to the arc of the Davie FZ with the proto-Owen fracture zone
sharing the same India-Madagascar vs Africa pole until Madagascar comes to rest (117.3 Ma in model
CR21BBHQ). There is thus still no need to invent further rifting between India and Madagascar in this
interval.
By about 117 Ma, Madagascar is at reast with respect to Africa while India continues southwards for a time.
This movement is pure strike-slip off NE Madagascar with dextral transtension along the longer join between
Madagascar and India. By about 100 Ma (Early Albian) this comes to a halt and India, Africa and Madagascar
form ‘North Gondwana’ while South America, Antarctica and Australia (‘South Gondwana’) move south at
similar velocities with only slight opening between Antarctica and South America at this time. (This gives rise
to the Anomaly-T hiatus recorded in spreading in the Weddell Sea). Note also that the well-defined
positions of Madagascar and Antarctica against Africa still leave little room for alternative positions for the
southern tip of India (and hence Sri Lanka) before Madagascar joins the Africa plate at about 117 Ma.
Constructing oceans that grow consistently around Sri Lanka has proved to be one of the most difficult parts
of the modelling process. Taken optimistically, the difficulty in finding a fully workable solution here
suggests that our present might be close to a unique and correct one. The exact position of Antarctica, once
the Cretaceous Quiet Zone has started, is probably the biggest source of uncertainty.
Late Cretaceous
Until about 100.5 Ma (GTS2020) – start Late Cretaceous, in the middle of the Cretaceous Quite Zone – we
have more dextral transtension between India and Madagascar to bring India to the more southerly position
from which it will start its rapid northwestward journey and to make space for the development of what will
become the Mascarene fragments within the rifted space between the two. Careful matching of fracture
zones off Sri Lanka and Antarctica at around 90 Ma suggests that this distance was still quite small (<150 km